In am sure , your doctor must have treated your child for worm infestations, which is a common cause of this type of symptoms. Lactose intolerance
is more likely to cause chronic diarrhea
rather than voracious appetite. Another thing which should be ruled out is wheat allergy
for which a blood test
-TTG is advised. I think you should consult a well experienced child specialist in your town as your child needs to be properly investigated for these symptoms.